Pressure compensator for a subsea device

ABSTRACT

A pressure compensator for a subsea device for performing a pressure compensation between an ambient medium surrounding the subsea device and a liquid medium filling a volume of the subsea device is provided. The pressure compensator has at least one outer bellow and a first chamber enclosed by the outer bellow. It further has at least one inner bellow which is arranged inside the first chamber, and a second chamber enclosed by the inner bellow. Between the outer bellow and the inner bellow, a compensation volume is confined, which is provided with a fluid connection to the volume of the subsea device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to EP Patent Application No. 11195885filed Dec. 28, 2011. The contents of which is incorporated herein byreference in its entirety.

TECHNICAL FIELD

The disclosure relates to a pressure compensator for a subsea device forperforming pressure compensation between an ambient medium surroundingthe subsea device and a liquid medium filling a volume of the subseadevice. The disclosure further relates to a subsea device comprisingsuch pressure compensator.

BACKGROUND

Due to the increasing energy demands, offshore oil and gas production ismoving into deeper waters. For ensuring an efficient and secureproduction, processing facilities are being installed at the oceanfloor. Such subsea installations can comprise a range of components,including pumps, compressors and the like as well as a power grid foroperating them. The power grid may for example comprise a subseatransformer, subsea switchgear and subsea variable speed drives. Thecomponents of the subsea installation need to be protected from thesurrounding sea water, in which pressures of 300 bar or more can prevail(at installation depths of 3.000 m or more).

Two solutions were proposed for dealing with these high pressures. Apressure resistant enclosure can be provided, which has a close toatmospheric internal pressure, enabling the use of conventional electricand mechanical components. Such enclosures need to have relatively thickwalls and are thus bulky and heavy, since they have to withstand highdifferential pressures.

Another solution is the use of pressurized (or pressure compensated)enclosures, which comprise a volume/pressure compensator which balancesthe pressure in the enclosure to the pressure prevailing in the ambientsea water.

The pressurized enclosure is generally filled with a liquid, andcomponents operated inside the pressurized enclosure are made to beoperable under high pressures. The pressure/volume compensatorcompensates for variations in the volume of the liquid filling theenclosure, which may occur due to variations in outside pressure and/ortemperature. Temperature changes can be cause by internal heating, e.g.due to electric losses.

Pressure compensators may include metal bellows, rubber bellows, pistonsor the like. Bellows can have the disadvantage that they are eitherexpensive to produce, or their configuration is such that the strokelength of the bellow is limited. In the latter case, a pressurecompensator for a large volume of liquid also requires large volume. Forsome types of bellows, the bellow needs to have a size of more thanthree times of the size of the compensated volume. This results in a lowutilization factor of the volume of the compensator system. Furthermore,the liquid filling such pressure compensator needs to be compensateditself. Such compensator systems can thus be relatively large and heavy.

In the document WO2010/034880A1, a pressure compensator is disclosedwhich has a first bellows chamber that is surrounded by a second bellowschamber, the second bellows chamber forming a closed intermediate spacearound the first bellows chamber. A double barrier against the ingressof sea water is thus obtained, but the pressure compensation capacity isthe same as if the first bellows chamber was provided by itself.

The document WO2011/088840A1 discloses a pressure compensation systemwhich achieves a double barrier against the ingress of sea water.

It is desirable to provide a pressure compensator for use with a subseadevice that can be manufactured easily and cost efficiently. It isfurther desirable that the pressure compensator provides security duringoperation and has a long lifetime. It is desirable to reduce the size ofthe pressure compensator, and to increase the utilization factor.

SUMMARY

In one embodiment, a pressure compensator is provided for a subseadevice for performing a pressure compensation between an ambient mediumsurrounding the subsea device and a liquid medium filling a volume ofthe subsea device, the pressure compensator comprising: at least oneouter bellow; a first chamber enclosed by said outer bellow; at leastone inner bellow, the inner bellow being arranged inside the firstchamber; a second chamber enclosed by the inner bellow; a compensationvolume confined between the outer bellow and the inner bellow; a firstfluid connection from the compensation volume towards the volume of thesubsea device, the first fluid connection being configured so as toenable the passage of the liquid medium between the compensation volumeand the volume of the subsea device; and a second fluid connection tothe second chamber, the second fluid connection being configured so asto enable the passage of the ambient medium into and out of the secondchamber.

In a further embodiment, the outer bellow comprises an outer cover plateand a bellow portion having two openings, the bellow portion beingliquid tight sealed against the outer cover plate at one opening andagainst a mounting plate which forms part of the outer bellow or of thesubsea device at the other opening. In a further embodiment, the innerbellow comprises an inner cover plate and a bellow portion having twoopenings, wherein the bellow portion of the inner bellow is liquid tightsealed against the inner cover plate at one opening and, at the otheropening, is liquid tight sealed against one of the mounting plate or theouter cover plate of the outer bellow. In a further embodiment, thebellow portion of the inner bellow is sealed against the mounting plate,and wherein the second fluid connection comprises one or more openingsin the mounting plate located within an area of the mounting plate thatis surrounded by the bellow portion of the inner bellow. In a furtherembodiment, the outer bellow comprises an outer cover plate, and whereinthe inner bellow comprises a bellow portion that is sealed against theouter cover plate, wherein the second fluid connection comprises one ormore openings in the outer cover plate located within an area of theouter cover plate that is surrounded by the bellow portion of the innerbellow. In a further embodiment, the outer bellow comprises a bellowportion, the bellow portion being liquid tight sealed against a mountingplate forming part of the outer bellow or of the subsea device, whereinthe first fluid connection comprises a fluid passage through themounting plate. In a further embodiment, the at least one outer bellowis a formed metal bellow. In a further embodiment, the at least oneinner bellow is a formed metal bellow. In a further embodiment, theformed metal bellow comprises a bellow portion that is made from a metalsheet formed into a cylindrical shape and provided with corrugationshaving crests which extend in circumferential direction of thecylindrical shape. In a further embodiment, the at least one outerbellow comprises a first outer bellow and a second outer bellow arrangedwithin the first outer bellow, wherein the first outer bellow and thesecond outer bellow confine a first intermediate volume, wherein thefirst outer bellow provides a liquid tight seal between the firstintermediate volume and the ambient medium, and wherein the second outerbellow provides a liquid tight seal between the first intermediatevolume and the compensation volume. In a further embodiment, the atleast one inner bellow comprises a first inner bellow and a second innerbellow arranged within the first inner bellow, wherein the first innerbellow and the second inner bellow confine a second intermediate volume,wherein the first inner bellow provides a liquid tight seal between thesecond intermediate volume and the compensation volume, and wherein thesecond inner bellow provides a liquid tight seal between the secondintermediate volume and the ambient medium. In a further embodiment, thepressure compensator comprises at least two inner bellows arrangedinside the first chamber, each of the two inner bellows enclosing achamber and comprising a fluid connection to enable the passage ofambient medium into the respective chamber, the at least two innerbellows being arranged adjacent to each other.

In a further embodiment, the pressure compensator further comprises: atleast a third bellow, the third bellow being arranged in the secondchamber and enclosing a third chamber, and a third fluid connectionbetween the compensation volume and the third chamber, the third fluidconnection being configured so as to enable the passage of the liquidmedium between the compensation volume and the third chamber.

In a further embodiment, the ambient medium is sea water, wherein theouter bellow is on its outer side in contact with the sea water when thepressure compensator is installed subsea, and wherein the second fluidconnection is configured to enable the passage of sea water into thesecond chamber so that the second chamber is filled with sea water whenthe pressure compensator is installed subsea.

In another embodiment, a subsea device comprises an enclosure enclosinga volume, the volume being filled with a liquid medium; and a pressurecompensator as disclosed above, the first fluid connection enabling theliquid medium to flow between the volume enclosed by the enclosure andthe compensation volume of the pressure compensator.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments will be explained in more detail below withreference to figures, in which:

FIG. 1 is a schematic diagram illustrating a conventional pressurecompensator coupled to a volume of a subsea device.

FIG. 2 is a schematic diagram illustrating a pressure compensatoraccording to one embodiment.

FIG. 3 is a schematic diagram illustrating a pressure compensatoraccording to a further embodiment.

FIGS. 4A and 4B are schematic diagrams illustrating pressurecompensators according to embodiments in which two inner bellowsarranged adjacent to one another are provided.

FIGS. 5A and 5B are schematic diagrams illustrating pressurecompensators according to embodiments in which a third bellow isprovided inside the inner bellow.

FIG. 6 is a schematic diagram illustrating a pressure compensatoraccording to an embodiment which is provided with a double barrieragainst the ambient medium.

FIGS. 7A, 7B and 7C are schematic diagrams illustrating different statesof a pressure compensator during pressure compensation according to oneembodiment.

FIGS. 8A and 8B are schematic diagrams showing a perspective view and aperspective sectional view of a pressure compensator in a particularimplementation of the embodiment of FIG. 2.

FIGS. 9A and 9B are schematic diagrams showing a perspective view and asectional perspective view, respectively, of a specific implementationof the pressure compensator of the embodiment of FIG. 3.

FIG. 10 is a schematic diagram illustrating a subsea device comprising apressure compensator in accordance with one embodiment.

DETAILED DESCRIPTION

Accordingly, embodiment of the present disclosure provide an improvedpressure compensator for subsea use.

One embodiment provides a pressure compensator for a subsea device forperforming a pressure compensation between an ambient medium surroundingthe subsea device and a liquid medium filling a volume of the subseadevice. The pressure compensator comprises at least one outer bellow, afirst chamber enclosed by the outer bellow, at least one inner bellow,the inner bellow being arranged inside the first chamber and a secondchamber enclosed by the inner bellow. Between the outer bellow and theinner bellow, a compensation volume is confined. The pressurecompensator further comprises a first fluid connection from thecompensation volume towards the volume of the subsea device, the firstfluid connection being configured so as to enable the passage of theliquid medium between the compensation volume and the volume of thesubsea device when the pressure compensator is installed on the subseadevice. A second fluid connection to the second chamber is furtherprovided, the second fluid connection being configured so as to enablethe passage of the ambient medium into and out of the second chamberwhen the pressure compensator is installed on a subsea device andsubmerged subsea.

In such configuration, the outer bellow and the inner bellow arearranged such that an expansion of the outer bellow and a compression ofthe inner bellow will lead to a volume increase of the compensationvolume. The pressure compensator may thus achieve higher volume changesof the compensation volume as compared to a pressure compensator havingonly an outer bellow of the same size. Further, the dead volume of theliquid medium is reduced, since the second chamber can be filled withthe ambient medium, in particular sea water, through the second fluidconnection, so that the utilization factor of the pressure compensatorcan be increased. Since the bellows are flexible and canexpand/contract, pressure differences between the ambient medium and theliquid medium can be compensated, i.e. the pressures of the ambientmedium and of the liquid medium in the compensation volume can bebalanced.

In an embodiment, the second fluid connection is between the secondchamber and the outside of the pressure compensator.

In an embodiment, the outer bellow comprises an outer cover plate and abellow portion having two openings. The bellow portion is sealed in aliquid tight manner against the outer cover plate at one opening andagainst a mounting plate at the other opening. The mounting plate may bepart of the outer bellow, or may be part of the subsea device. As anexample, the liquid tight seal may be achieved by welding. The bellowportion may have a cylindrical shape, with openings at the top andbottom.

The inner bellow may comprise an inner cover plate and a bellow portionhaving two openings, wherein the bellow portion of the inner bellow maybe sealed in a liquid tight manner against the inner cover plate at oneopening. At the other opening, it may be sealed against the mountingplate or the outer cover plate of the outer bellow. Again, the bellowportion may be cylindrical and may have openings at its top and bottom,and the liquid tight sealing may be achieved by welding.

The configuration of the pressure compensator may be such that the outercover plate and the inner cover plate are free to move in axialdirection of the respective bellow portions, the movement of the outercover plate being independent from the movement of the inner coverplate. In such configuration, large variations of the compensationvolume can be achieved, while at the same time, the absolute size of thecompensation volume can be kept small.

In an embodiment, the bellow portion of the inner bellow is sealedagainst the mounting plate, and the second fluid connection comprisesone or more openings in the mounting plate located within an area of themounting plate that is surrounded by the bellow portion of the innerbellow. The one or more openings may for example comprise a singleopening having a diameter that is only slightly smaller than thediameter of the bellow portion of the inner bellow. Ambient medium canthus easily enter the second chamber.

In a further embodiment, the outer bellow comprises an outer coverplate, and the inner bellow comprises a bellow portion that is sealedagainst the outer cover plate, wherein the second fluid connectioncomprises one or more openings in the outer cover plate located withinan area of the outer cover plate that is surrounded by the bellowportion of the inner bellow. In such configuration, the entering of theambient medium into the second chamber is facilitated even if thepressure compensator is directly mounted onto an enclosure of the subseadevice, e.g. via the mounting plate.

The outer bellow may comprise a bellow portion, the bellow portion beingliquid tight sealed against a mounting plate forming part of the outerbellow or of the subsea device, wherein the first fluid connectioncomprises a fluid passage through the mounting plate. This way, the flowof the liquid medium between the volume of the subsea device and thecompensation volume can be facilitated, for example when the mountingplate forms part of an enclosure of the subsea device, or is directlymounted to such enclosure.

The fluid passage may comprise or consist of a conduit, a duct, achannel, a pipe, a tubing or the like, it comprises a penetrationthrough the mounting plate enabling the flow of liquid therethrough.

In some embodiments, the at least one outer bellow and/or the at leastone inner bellow is a formed metal bellow. Using a formed metal bellowmay make the pressure compensator robust and furthermore cost-efficientto manufacture. By the configuration of the pressure compensator,disadvantages which may result from a limited stroke of a formed metalbellow may be mitigated, since the pressure compensator achieves largechanges of the compensation volume and a high utilization factor evenwith bellows having limited stroke.

The formed metal bellow may comprise a bellow portion that is made froma metal sheet formed into a cylindrical shape and provided withcorrugations. The crests of the corrugations may extend incircumferential direction of the cylindrical shape, so that anexpansion/compression of the bellow portion in its axial directionbecomes possible. The bellow may thus be referred to as a corrugatedbellow.

In an embodiment, the at least one outer bellow comprises a first outerbellow and a second outer bellow arranged within the first outer bellow.The first outer bellow and the second outer bellow confine a firstintermediate volume. The first outer bellow provides a liquid tight sealbetween the first intermediate volume and the ambient medium and thesecond outer bellow provides a liquid tight seal between the firstintermediate volume and the compensation volume. In this configuration,a double barrier against the ambient medium can be provided at the outerbellow. Each outer bellow can comprise a bellow portion and a coverplate, both bellow portions being mounted to the same mounting plate.The cover plates may be configured to move freely and independently fromeach other. This way, the intermediate volume can be filled with aliquid, which is pressure compensated both against the ambient mediumand the liquid medium in the compensation volume.

Similarly, the at least one inner bellow may comprise a first innerbellow and a second inner bellow arranged within the first inner bellow.The first inner bellow and the second inner bellow may confine a secondintermediate volume. The first inner bellow may provide a liquid tightseal between the second intermediate volume and the compensation volume,and the second inner bellow may provide a liquid tight seal between thesecond intermediate volume and the ambient medium. Each of the first andsecond inner bellows may comprise a cover plate and a bellow portion,both bellow portions being mounted to the same mounting plate of theouter bellow, or to the cover plate of the outer bellow (in particularthe second outer bellow). The cover plates of both inner bellows mayagain be configured to move freely and independently from each other. Adoubled barrier against the ingress of the ambient medium which may fillthe second chamber when the pressure compensator is installed subsea isthus provided. The second intermediate volume can be filled with aliquid which is pressure compensated against the ambient medium andagainst the liquid medium in the compensation volume.

By implementing both of the above configurations, a full double barriersealing of the compensation volume against the ambient medium can beachieved. Ambient medium will only enter the intermediate volume, shouldone of the bellows being in contact with the ambient medium fail.

In an embodiment, the pressure compensator may comprise at least twoinner bellows arranged inside the first chamber, each of the two innerbellows enclosing a chamber and comprising a fluid connection to enablethe passage of ambient medium into the respective chamber, the at leasttwo inner bellows being arranged adjacent to each other. In someapplications, such configuration may further increase the utilizationfactor.

The pressure compensator may comprise at least a third bellow, the thirdbellow being arranged in the second chamber and enclosing a thirdchamber. A third fluid connection may be provided between thecompensation volume and the third chamber, the third fluid connectionbeing configured so as to enable the passage of the liquid mediumbetween the compensation volume and the third chamber. Volume changes ofthe compensation volume which can be achieved with the pressurecompensator may be made even larger in such configuration. This isparticularly advantageous when using formed metal bellows having alimited stroke.

The ambient medium may be sea water, and the outer bellow may on itsouter side be in contact with the sea water when the pressurecompensator is installed subsea. The second fluid passage may beconfigured to enable the passage of seawater into the second chamber sothat the second chamber is filled with seawater when the pressurecompensator is installed subsea. The compensation volume may be filledwith a liquid, in particular with a dielectric liquid, such as oil.

A further embodiment provides a subsea device comprising an enclosureenclosing a volume, the volume being filled with a liquid medium and apressure compensator in any of the above described configurations. Thefirst fluid connection of the pressure compensator enables the liquidmedium to flow between the volume enclosed by the enclosure and thecompensation volume of the pressure compensator. In particular whenusing a configuration of the pressure compensator having formed metalbellows, pressure compensation of the volume of the subsea device can beachieved in a cost-efficient manner, while the size of the dead volumeof the pressure compensator can be reduced.

The bellow portion of the outer bellow of the pressure compensator mayhave a cylindrical shape having a diameter in a range between about 0.5m and about 2.5 m. In uncompressed/unextended state, the bellow portionmay furthermore have a height between about 0.5 m and about 2.5 m. Theinner bellow may be sized such that the ratio of the volume enclosed bythe inner bellow and the volume enclosed by the outer bellow lies withina range of about 0.3 to about 0.9.

In the following, the embodiments illustrated in the accompanyingdrawings are described in more detail. It should be clear that thefollowing description is only illustrative and non-restrictive. Thedrawings are only schematic representations, and elements in thedrawings are not necessarily to scale with each other.

FIG. 1 illustrates a subsea system 100 comprising a conventionalpressure compensator 101. The illustrated conventional pressurecompensator 101 consists of a single bellow confining a compensationvolume which is connected via pipe 103 to the inner volume of a subseaenclosure 102. In particular when using a formed metal bellow in thepressure compensator 101, the stroke of the bellow is rather limited,resulting in a large absolute volume of pressure compensator 101 for agiven volume variation that is to be achieved by the pressurecompensator. Pressure compensator 101 thus requires a lot of space and,due to its size, is relatively heavy. It needs to be filled with a largevolume of liquid, which itself needs to be compensated for volumevariations. In particular, the bellow of pressure compensator 101 may berequired to have a size of more than three times of the compensatedvolume in enclosure 102, due to the low utilization factor of thecompensator. The utilization factor to the total volume of the pressurecompensator may be defined as the ratio of the achievable volumevariation.

FIG. 2 shows a sectional side view of a pressure compensator 50 inaccordance with one embodiment. Pressure compensator 50 comprises afirst bellow 10 including a bellow portion 11, the upper part of whichis sealed against the cover plate 12 and the lower part of which issealed against the mounting plate 13. A liquid tight seal may beprovided by welding the bellow portion 11 to the respective plate. Inthe mounting plate 13, a first fluid connection 15 is provided. Fluidconnection 15 provides a fluid communication between the volume enclosedby the outer bellow 10 and a volume enclosed by an enclosure of a subseadevice, for which the pressure compensator 50 provides a volume/pressurecompensation. First fluid connection 15 may be a simple throughconnection, i.e. a penetration of the mounting plate 13, or it maycomprise a pipe, a tube, a duct, a channel, a conduit or the like.

Pressure compensator 50 further comprises an inner bellow 20 which isarranged in a chamber enclosed by the outer bellow 10. The inner bellow20 comprises a bellow portion 21 which is sealed against the inner coverplate 22 of inner bellow 20 and against the outer cover plate 12 of theouter bellow 10. A liquid tight seal between the bellow portion 21 andthe respective cover plate may again be obtained by welding.

A second fluid connection 25 is provided between a second chamber 24enclosed by the inner bellow 20 and the outside of pressure compensator50. The second fluid connection 25 is configured so that an ambientmedium surrounding the pressure compensator 50, such as seawater, canenter through the second connection 25 into the second chamber 24. Dueto the liquid tight seals provided at the periphery of the bellowportion 21, seawater can not enter into the compensation volume 14,which is confined between the inner bellow 20 and the outer bellow 10.

The outer bellow 10 encloses a first chamber, in which the inner bellow20 is arranged. Due to the liquid tight seals provided at the peripheryof the outer bellow portion 11 and the inner bellow portion 21, a volumeis confined between the respective bellow portions, the outer coverplate 12, the inner cover plate 22 and the mounting plate 13. Thisconfined compensation volume 14 is available for pressure compensation,it can be varied considerably by extension and compression of the outerand inner bellows. The compensation volume 14 is filled with a liquid,in particular a dielectric liquid, which can enter and flow out of thecompensation volume 14 via the first fluid connection 15. Ambientmedium, such as seawater, can not enter the compensation volume 14.

An enclosure of a subsea device encloses a volume in which for examplean electric component can be arranged. The volume is in flowcommunication with the compensation volume 14 via the first fluidconnection 15. If the liquid in this volume of the subsea enclosureexpands, for example due to a rise in temperature, the liquid can enterthe compensation volume 14. By an expansion of outer bellow 10 and/or acompression of the inner bellow 20, the compensation volume 14 isenlarged, so that the additional liquid can be taken up by the pressurecompensator 50 without causing a drastic increase in pressure. Inanother example, if the ambient pressure around pressure compensator 50increases, for example when installing a subsea device to which pressurecompensator 50 is mounted at great water depths, the volume of theliquid will decrease due to the increasing pressure and/or due to thedecreasing temperature. The decreasing volume of the liquid can becompensated by a compression of outer bellow 10 and an extension ofinner bellow 20, leading to a decreasing compensation volume 14. By theflexible bellow portions 11, 21, any pressure changes in the ambientmedium are directly transduced to the liquid filling compensation volume14, so that the pressure of the liquid is balanced to the pressure ofthe ambient medium. Note that a slight overpressure may be maintained inthe compensation volume 14, for example by biasing (pre-loading) thebellows or the like. Biasing may for example occur by the weight of thecover plates, by a spring applying a force to a bellow, by using thebellow as a spring, or by any other means. A slight overpressure willprevent the entering of ambient medium into the volume of the subseadevice, since a slight overpressure of the liquid will result in theliquid being pushed out through any leak. This also facilitates thedetection of a leak.

In the arrangement of FIG. 2, pressure compensator 50 has thecompensation capacity of both the inner bellow and the outer bellow,without requiring more space than the outer bellow 10 itself. Also, thevolume of liquid required in the compensation volume 14 is relativelysmall, while relatively large volume changes are enabled. The pressurecompensator 50 of FIG. 2 can thus make use of formed metal bellows,which are easy and cost-efficient to manufacture, while the total sizeof the pressure compensator that is required for a particular volume ofthe subsea device can be kept comparatively small.

In the embodiments described herein, the outer bellow 10 and the innerbellow 20 are formed metal bellows, although it should be appreciatedthat the described configurations may also be employed with other typesof bellows.

The formed metal bellow 10, 20 may be manufactured by using a sheet ofmetal, the peripheral edges of which are welded together so as to form acylinder. The corresponding bellow portion may be obtained by hydroforming the metal sheet or the cylinder, e.g. using templates for thedesired shape of the bellow portion. Another possibility is the use of arolling technique in which the metal cylinder is deformed mechanicallyto form the corrugations of the bellow portion. Whether the corrugationsare provided by hydroforming or by rolling can depend on the diameter ofthe metal cylinder. For larger diameters, e.g. above 1200 mm diameter,corrugations may be formed by rolling while below this diameter, theymay be hydro formed.

The formed bellows may thus also be termed corrugated bellows. Comparedto welded bellows, in which the corrugations are formed by welding,corrugated bellows can be more reliable in subsea applications, they areless prone to corrosion and have a higher life expectancy in repeatedcompression/extension cycles.

The formed metal bellow may have a stroke length of between about 10%and about 20% of its length in each way, giving a total stroke length ofbetween about 20 to about 40%. As an example, from its operatingopposition, the bellow may be stretched or compressed 15% each way. Theintrinsic spring constant of the formed metal bellow may be used togenerate the above mentioned slight overpressure inside the compensationvolume 14.

The outer bellow of pressure compensator 50 may for example have adiameter between about 0.5 m and about 2.5 m. Note that these are onlyexamples, and that pressure compensators having other diameter bellowsmay be provided.

The remaining figures show modifications of the pressure compensator 50,which are explained in more detail hereinafter. Accordingly, the aboveobservations and explanations apply equally to the embodiments describedhereinafter.

In the embodiment of FIG. 3, the inner bellow 20 is not sealed to thecover plate 12, but to the mounting plate 13. Correspondingly, thesecond fluid connection 25 is provided in the mounting plate 13. Thesecond fluid connection 25 is now at the lower side of the pressurecompensator 50, which can have advantages for certain configurations ofthe subsea device towards which pressure compensator 50 is to beconnected.

FIGS. 4A and 4B show embodiments of the pressure compensator 50 in whichtwo inner bellows 20, 30 are provided and arranged adjacent to eachother. In FIG. 4A, the two inner bellows 20, 30 are sealed against theouter cover plate 12 of the outer bellow 10. For each inner bellow 20,30, a second fluid connection 25, 35 is provided. Each inner bellowfurther comprises its own inner cover plate 22, 32, which can moveindependent from one another. It should be clear that more than two,e.g. three, four, five, . . . inner bellows may be provided in otherconfigurations.

In FIG. 4 b, the two inner bellows 20, 30 are sealed against themounting plate 13. The configuration is thus similar to the one of FIG.3, but with two inner bellows. It should be clear that thisconfiguration may also have more than two inner bellows, and that theconfigurations of FIGS. 4 a and 4 b may be mixed, e.g. by having one ormore bellows sealed against the cover plate 12 and by having one or morebellows sealed against the mounting plate 13.

In the configurations of FIGS. 5 a and 5 b, the pressure compensator 50further comprises a third bellow 40, which is arranged inside thechamber 24 enclosed by the inner bellow 20. Furthermore, a third fluidconnection 45 is provided between the chamber 44 enclosed by the thirdbellow 40 and the compensation volume 14. In both embodiments of FIGS. 5a and 5 b, the third bellow 40 is sealed against the inner cover plate22 of the inner bellow 20, with the third fluid connection 45 beingprovided in form of an opening in the cover plate 22. The configurationof FIG. 5 a corresponds to the one of FIG. 2, whereas the configurationof FIG. 5 b corresponds to the one of FIG. 3. In both configurations,the third bellow 40 further increases the volume variations that arepossible by means of pressure compensator 50. This is due to thepossibility to compress or extend the third bellow 40, therebyincreasing or decreasing the volume of the chamber enclosed by the thirdbellow. Since this chamber is in fluid communication with thecompensation volume 14, it allows for larger volume variations of theliquid filling the compensation volume 14 and the volume of the subseadevice.

FIG. 6 shows an embodiment the configuration of which corresponds to theone of FIG. 3. In the embodiment of FIG. 6, a double barrier design isimplemented. The outer bellow 10 comprises a first outer bellow 61 and asecond outer bellow 62, each having a bellow portion and an outer coverplate. Between these bellow portions, the outer cover plates and themounting plate 13, an outer intermediate volume 63 is confined. It issealed against the compensation volume 14 and against the ambient mediumsurrounding pressure compensator 50. Similarly, the inner bellow 20comprises a first inner bellow 71 and a second inner bellow 72. Thesecond inner bellow 72 is arranged within the first inner bellow 71,each bellow comprising an inner cover plate. Between the inner coverplates and the bellow portions of the first and second inner bellows 71,72 and the mounting plate 13, an inner intermediate volume 73 isconfined.

The liquid in the compensation volume 14 is now protected againstambient medium surrounding the pressure compensator 50 or which mayenter the second chamber 24 of the inner bellow 20 by a double barrier.Ambient medium leaking through the first outer bellow 61 or the secondinner bellow 72 will thus be confined in the outer intermediate volume63 or the inner intermediate volume 73, respectively, and will not enterthe compensation volume 14. It should be clear that such double barrierconfiguration may also be provided in any of the other embodimentsdescribed above or further below.

FIGS. 7 a to 7 c illustrate with respect to the configuration of FIG. 3the operation of the pressure compensator 50. FIG. 7 a shows theequilibrium operating state, which may for example be achieved when thesubsea device comprising pressure compensator 50 is installed subsea ata nominal operating depth. Both the inner bellow 20 and the outer bellow10 have the capability of being compressed or stretched, and the liquidwithin compensation volume 14 may have a slight overpressure as comparedto seawater (SW) surrounding pressure compensator 50 and entering thesecond chamber 24. Starting from the state as illustrated in FIG. 7 a, asituation is now depicted in FIG. 7 b in which the ambient pressure isdropped or the temperature is increased, resulting in an increase of thevolume of the liquid in the subsea device. Due to the volume expansion,liquid enters the compensation volume 14 through the first fluidconnection 15, which leads to a stretching of the outer bellow 10 and toa compression of the inner bellow 20, as illustrated by arrows. As thevolume of the second chamber 24 is decreased, seawater is pushed outthrough the second fluid connection 25.

The pressure of the liquid inside compensation volume 14 applies a forcein the direction of the arrows to the outer cover plate and to the innercover plate, which is balanced by the force applied to these plates bythe pressure of the ambient medium. Consequently, pressure compensator50 provided a pressure balancing between the pressure of the ambientmedium and the pressure inside compensation volume 14 (except for theabove mentioned pressure biasing loading, if such is provided). Sincethe pressure balancing is achieved by compensating volume differences ofthe liquid, the pressure compensator 50 may also be termed “volumecompensator”.

In FIG. 7 c, a situation is illustrated in which the volume of theliquid in the subsea device is decreased, e.g. due to an increase inambient pressure or due to a decrease in temperature of the liquid.Consequently, liquid flows from the compensation volume 14 through thefirst fluid connection 15 into the volume of the subsea device. Thecompensation volume is correspondingly decreased by a compression of theouter bellow 10 and an expansion of the inner bellow 20 as indicated byarrows. The expansion of inner bellow 20 leads to seawater entering thesecond chamber 24 through the second fluid connection 25. Again, bychanging compensation volume 14, the pressure of the liquid is equalizedto the ambient pressure.

FIGS. 8 a and 8 b show a particular implementation of the pressurecompensator 50 in the configuration of FIG. 2. In the perspective viewof FIG. 8 a, it can be seen that the second fluid connection 25comprises several openings in the outer cover plate 12. Furthermore, thefirst fluid connecting 15 is provided in form of a pipe or hose from themounting plate 13. This can also be seen from the sectional perspectiveview of FIG. 8 b, showing the bellow portions 11 and 21 of the outerbellow 10 and the inner bellow 20, respectively, in more detail.Providing several openings in cover plate 12 facilitates the entering ofambient medium into the second chamber 24 of the inner bellow 20.

FIGS. 9 a and 9 b show a particular implementation of the pressurecompensator 50 in the configuration of FIG. 3. As can be seen from theperspective view of FIG. 9 a, the outer cover plate 12 is not providedwith openings, which may for example prevent debris from entering thesecond chamber 24. The sectional perspective view of FIG. 9 b shows thatthe mounting plate is provided with an opening providing the secondfluid connection 25. The opening is surrounded by the bellow portion 21of the inner bellow 20, it has a size that essentially corresponds tothe diameter of the inner bellow portion 21. The flow of ambient mediuminto the second chamber surrounded by the second bellow 20 is thusessentially non-restricted. The first fluid connection 15 is againprovided through the mounting plate 13, it is implemented as pipe or ahose in the example of FIG. 9.

Note that while in all of the configurations described above, the firstfluid connection 15 is implemented via a penetration of the mountingplate 13, it is certainly also conceivable to provide the first fluidconnection at another position, for example through the cover plate 12.Since cover plate 12 moves during pressure compensation, it isbeneficial to provide the first fluid connection in the mounting plate13.

FIG. 10 illustrates a subsea device 80 in accordance with oneembodiment. Subsea device 80 comprises an enclosure 81 enclosing avolume 82 which is filled with liquid, in particular dielectric liquid.An electric component 83 is submersed in the liquid filling volume 82.Subsea device 80 further comprises a pressure compensator 50. Note thatalthough in FIG. 10, pressure compensator 50 has the configuration asillustrated in FIG. 2, any of the configurations of the pressurecompensator 50 described herein may be used in the subsea device 80. Theliquid in volume 82 can enter the compensation volume 14 of pressurecompensator 50 through the first fluid connection 15. The ambient medium90 which surrounds the subsea device 80 when installed subsea, inparticular seawater, can enter the second chamber 24 of the inner bellow20 via the second fluid connection 25.

Mounting plate 13 can be a metal plate which is part of the pressurecompensator 50, in particular part of the outer bellow 10. In otherconfigurations, mounting plate 13 may be part of the enclosure 81 ofsubsea device 80, it may in particular be a wall of the enclosure 81. Insome configurations, the mounting plate 13 may thus be mounted to a wallof enclosure 81, e.g. by bolting or welding mounting plate 13 onto thewall, while in other configurations, the bellow portion 11 may bedirectly welded to a wall of enclosure 81.

The first fluid connection 15 may be a simple opening in the mountingplate 13 and the respective wall of the enclosure 81, i.e. a penetrationthrough the material layers separating the compensation volume 14 fromvolume 82. As described further above, other embodiments may use a pipeor tubing for connecting the compensation volume 14 to volume 82.

In operation, the mounting plate 13 is generally stationary, as it ismounted to the enclosure of the subsea device, whereas the cover plateis moving during volume compensation. The outer cover plate 12 and theinner cover plate 22 move independent from each other, and generally inopposite direction during operation, as outlined with respect to FIGS. 7a to 7 c.

The above embodiments provide a pressure compensator that iscost-efficient to produce, while at the same time provides a highutilization factor and has only a limited size and weight. Theutilization factor of the pressure compensator may be increased to about60%, or even more. The volume of unused liquid can be reduced at thesame time. Since formed metal bellows can be employed in the pressurecompensator, resistivity to corrosion and an extended lifetime arefurther benefits that can be achieved.

What is claimed is:
 1. A pressure compensator for a subsea device forperforming a pressure compensation between an ambient medium surroundingthe subsea device and a liquid medium filling a volume of the subseadevice, the pressure compensator comprising: at least one outer bellow;a first chamber enclosed by said outer bellow; at least one innerbellow, the inner bellow being arranged inside the first chamber; asecond chamber enclosed by the inner bellow; a compensation volumeconfined between the outer bellow and the inner bellow; a first fluidconnection from the compensation volume towards the volume of the subseadevice, the first fluid connection enabling passage of the liquid mediumbetween the compensation volume and the volume of the subsea device; anda second fluid connection to the second chamber, the second fluidconnection enabling passage of the ambient medium into and out of thesecond chamber.
 2. The pressure compensator of claim 1, wherein theouter bellow comprises an outer cover plate and a bellow portion havingtwo openings, the bellow portion being liquid tight sealed against theouter cover plate at one opening and against a mounting plate whichforms part of the outer bellow or of the subsea device at the otheropening.
 3. The pressure compensator of claim 2, wherein the innerbellow comprises an inner cover plate and a bellow portion having twoopenings, wherein the bellow portion of the inner bellow is liquid tightsealed against the inner cover plate at one opening and, at the otheropening, is liquid tight sealed against one of the mounting plate or theouter cover plate of the outer bellow.
 4. The pressure compensator ofclaim 3, wherein the bellow portion of the inner bellow is sealedagainst the mounting plate, and wherein the second fluid connectioncomprises one or more openings in the mounting plate located within anarea of the mounting plate that is surrounded by the bellow portion ofthe inner bellow.
 5. The pressure compensator of claim 1, wherein theouter bellow comprises an outer cover plate, and wherein the innerbellow comprises a bellow portion that is sealed against the outer coverplate, wherein the second fluid connection comprises one or moreopenings in the outer cover plate located within an area of the outercover plate that is surrounded by the bellow portion of the innerbellow.
 6. The pressure compensator of claim 1, wherein the outer bellowcomprises a bellow portion, the bellow portion being liquid tight sealedagainst a mounting plate forming part of the outer bellow or of thesubsea device, wherein the first fluid connection comprises a fluidpassage through the mounting plate.
 7. The pressure compensator of claim1, wherein the at least one outer bellow is a formed metal bellow. 8.The pressure compensator of claim 1, wherein the at least one innerbellow is a formed metal bellow.
 9. The pressure compensator of claim 7,wherein the formed metal bellow comprises a bellow portion that is madefrom a metal sheet formed into a cylindrical shape and provided withcorrugations having crests which extend in circumferential direction ofthe cylindrical shape.
 10. The pressure compensator of claim 1, whereinthe at least one outer bellow comprises a first outer bellow and asecond outer bellow arranged within the first outer bellow, wherein thefirst outer bellow and the second outer bellow confine a firstintermediate volume, wherein the first outer bellow provides a liquidtight seal between the first intermediate volume and the ambient medium,and wherein the second outer bellow provides a liquid tight seal betweenthe first intermediate volume and the compensation volume.
 11. Thepressure compensator of claim 1, wherein the at least one inner bellowcomprises a first inner bellow and a second inner bellow arranged withinthe first inner bellow, wherein the first inner bellow and the secondinner bellow confine a second intermediate volume, wherein the firstinner bellow provides a liquid tight seal between the secondintermediate volume and the compensation volume, and wherein the secondinner bellow provides a liquid tight seal between the secondintermediate volume and the ambient medium.
 12. The pressure compensatorof claim 1, comprising at least two inner bellows arranged inside thefirst chamber, each of the two inner bellows enclosing a chamber andcomprising a fluid connection to enable the passage of ambient mediuminto the respective chamber, the at least two inner bellows beingarranged adjacent to each other.
 13. The pressure compensator of claim1, further comprising: at least a third bellow, the third bellow beingarranged in the second chamber and enclosing a third chamber, and athird fluid connection between the compensation volume and the thirdchamber, the third fluid connection being enabling passage of the liquidmedium between the compensation volume and the third chamber.
 14. Thepressure compensator of claim 1, wherein the ambient medium is seawater, wherein the outer bellow is on its outer side in contact with thesea water when the pressure compensator is installed subsea, and whereinthe second fluid connection is configured to enable the passage of seawater into the second chamber so that the second chamber is filled withsea water when the pressure compensator is installed subsea.
 15. Asubsea device, comprising: an enclosure enclosing a volume filled with aliquid medium; and a pressure compensator for performing a pressurecompensation between an ambient medium surrounding the subsea device andthe liquid medium filling the volume, the pressure compensatorcomprising: at least one outer bellow; a first chamber enclosed by saidouter bellow; at least one inner bellow, the inner bellow being arrangedinside the first chamber; a second chamber enclosed by the inner bellow;a compensation volume confined between the outer bellow and the innerbellow; a first fluid connection from the compensation volume towardsthe volume of the subsea device, the first fluid connection enablingpassage of the liquid medium between the compensation volume and thevolume of the subsea device; and a second fluid connection to the secondchamber, the second fluid connection enabling passage of the ambientmedium into and out of the second chamber.